feat: add FetchIDL method to fetch idl json data

[gtsigner]

Add a method program.FetchIDL same as here https://github.com/project-serum/anchor/blob/master/ts/src/program/index.ts

[gagliardetto]

hey @godtoy

being able to fetch IDLs from the chain is a very useful thing to have! 😄

from a first look, it looks good to me.

but can you please use the standard go formatter?

[gtsigner]

Do u mean the code formatter ? should i exec go fmt ... first right?

[gtsigner]

Hey i just finished go fmt ... submit

[gagliardetto]

The tests fail because the IDL has been updated 😶‍🌫️

[gagliardetto]

How about changing the test to something like this:

• Instead of asserting a size equality, how about asserting json content field? How about idlJSON["name"] == "nft_magiceden"?

[gtsigner]

Yep, u are right. Because magiceden contract has been updated 2 days ago.